The present invention relates to a hologram recording and reproduction devices and methods for multiple recording of an interference fringe of a reference beam and a signal beam on a hologram recording medium by an angle multiplex system, particularly to the setting of the angular pitch of the angle of incidence of the reference beam on the hologram recording medium.
As described, for example, in IBM J. RES. DEVELOP Vol. 44, No. 3, May 2000, “Holographic data storage”, holographic data storage is the technology of disposing a hologram recording medium in a region where two luminous fluxes of a signal beam and a reference beam interfere with each other to thereby record an interference fringe of the two luminous fluxes on the hologram recording medium. In this case, for enhancing the recording density on the hologram recording medium, multiple recording of holograms is performed by any of various multiplexing systems including not only angle multiplex system but also shift multiplex system, speckle multiplex system, and phase encoding multiplex system.
In any of the multiplexing systems, the basic configuration of the hologram recording/reproduction device can be deemed substantially the same, except for the kind of the reference beam wave front which determines the principle of multiplexing. As a representative of these systems, there is the holographic storage recording/reproduction device based on the angle multiplex system. In this technology, the hologram recording medium is irradiated with two luminous fluxes of a signal beam and a reference beam, and the angle of incidence of the reference beam on the hologram recording medium is varied each time of spatial modulation (intensity modulation) of the signal beam according to a data page displayed on a spatial modulator (SLM) to thereby achieve multiple recording of holograms in the same recording region. Recorded on the hologram recording medium are Fourier images of two-dimensional bit data images (data pages) displayed on the SLM. In addition, the reproduced signals are data page images obtained by inverse Fourier transform of a diffracted beam, which are picked up by an image pickup device such as CCD, and are taken into a computer as bit patterns.
The multiple recording by the angle multiplex system is including the reference beam constituted of a plane wave is reflected by a rotating mirror (or galvano mirror) to vary the angle of incidence on the recording medium, whereby a number of sheets of holograms can be multiply recorded in the same region. This is made possible by the angle selectivity which is a characteristic feature of the volume hologram.
Here, the angle selectivity means the phenomenon in which at the time of reproducing a certain single sheet of hologram, the diffraction efficiency shows a behavior according to a Sin c function with the amount of deviation of the reproduction reference beam from the recording reference beam angle as a variable. To be more specific, let the recording wavelength of the hologram be λ, let the thickness of the recording medium (recording material) be T, the incidence angles of the signal beam and the reference beam be respectively θo=θr=θ, then the diffraction efficiency η varies according to the following formula in relation to the angle variation Δθ of the reproduction reference beam:η∝sin c2(2T(Δθ)sin θ/λ)  (1)
For example, the relationship between the diffraction efficiency of holograms recorded under the conditions of a recording wavelength of 532 nm, a recording medium thickness of 1 mm, and signal beam and reference beam incidence angles of 20 degrees and the variation in the incidence angle of the reproduction reference beam behaves as indicated by the characteristic curve plotted in FIG. 7.
In addition, the angle variation at the time when the diffraction efficiency becomes zero for the first time in the process in which the angle deviation of the reproduction reference beam is gradually increased from zero is determined by the wavelength λ of the light source used for recording, the thickness T of the recording medium (recording material), the incidence angle θo of the signal beam, and the incidence angle θr of the reference beam, and is represented by the following formula:Δθ=(Δθ)B=λ/T(sin θr+sin θo ·cos θr/cos θo)  (2).It should be noted here that all the angles shown hereinabove are values in the recording material.
Now, the case where the incidence angle of the reference beam varies at the time of multiple recording of holograms is considered. For example, in the case where the refractive index of a photo-polymer is 1.5, the incidence angle of the signal beam in air is 15 degrees, and the incidence angle of the reference beam is varied from 35 degrees to 65 degrees, Δθ can be determined from the formula (2), and has a characteristic as shown in FIG. 8; thus, Δθ decreases as the incidence angle of the reference beam increases.
The table shown in FIG. 9 shows the variation in the incidence angle of the reference beam at the time of recording a plurality of data pages in a single recording region by the angle multiplex system. The variation in the incidence angle of the reference beam is Δθ, which is constant. Therefore, the movable range of the reference beam is represented as:θtotal=(M−1)×Δθ  (3)